A computer disk drive typically employs a servo system containing means to position a read/write head relative to a data-containing disk. The disk has data arranged in tracks which also contain a track following signal. When it is desired to move the read/write head to a new track, a broadband signal is applied to the positioning means to drive the read/write head to the desired track.
As the read/write head nears the desired track, a reverse polarity signal is applied to the positioning means, causing it to act like a brake and ideally stopping the read/write head over the desired track, centered over the track following signal. During this process, considerable but variable heat is generated. Furthermore the mechanical structure of the positioning means has a natural resonance at a frequency f, the value of which is a function of temperature. Thus frequency f may range from a low of f1 to a high of f2.
The prior art solution to the problem involved attenuating the broadband signal over the entire range of possible resonant frequencies, i.e., the creation of a notch in the frequency spectrum of the broadband signal. Any such attenuation also affects the broadband signal at frequencies away from the notch, and results in a less than ideal behavior. By widening the notch to account for the changes in frequency of the resonant mode due to temperature, this effect is increased. Further, as the depth of the notch is not constant with frequency (that is, every notch is more effective at some frequencies than others), the behavior of the system can change as temperature changes move the frequency of the resonant mode.
In summary, the problems with the prior art solution are (1) the notch needs to be wide to account for the change in frequency of the resonant mode with temperature, and (2) the behavior of the system can change as temperature changes move the frequency of the resonant mode.
Accordingly, it is an object of this invention to provide an improved method and apparatus for control of a read/write head positioning means under conditions of varying temperature.